In the fabrication of high quality printed circuit boards, it is necessary to coat the substrate with a tough, resistant coating in selected areas which serves as a solder mask. In high quality printed circuit boards, molten solder is used to insure good interconnection between the components and the circuitry.
The coating to be used for such applications must possess a number of important characteristics. In particular, the coatings must be strongly adherent to a wide variety of metallic and non-metallic substrates, and resistant to harsh environmental conditions such as high humidity and elevated temperatures. The coatings also must be impervious to a wide variety of solvents, resistant to a large number of different chemicals, and particularly be resistant to commonly-used flux compositions and solders.
Moreover, since it is preferred that the solder masks remain on the circuitry as a protective coating, such must possess electrical insulating properties which last over long periods of time. Also the coatings must be durable and possess the above properties even after long periods of exposure to a wide variety of chemicals and adverse environmental conditions. The solder masks are left on the circuitry in order to protect it from oxidation and from the adverse effects of such gases as sulfur dioxide, hydrogen sulfide and chlorine.
The compositions used in producing these coatings, to be used in screen printing processes, must also be capable of good pattern definition which is dependent to a large extent upon the flow characteristics of the composition. Basically, the screen printing technique involves squeezing a composition through the open meshes of a streched piece of material such as wire onto a printable substrate. The screen is covered or blocked out in part by a masking material in order to form the desired pattern on the printable substrate. The masking material may simply be a stencil or a dried lacquer, shellac or glue. Once the screen has been covered or blocked out in part by a masking material, it is held taut on a frame and positioned over the desired substrate. The coating composition is then poured onto the screen and squeezed through the open areas with a squeegee. Thereafter, the frame is removed and the coated substrate baked. It is important, when following a desired pattern, that the composition do not flow or bleed outside of the preselected areas defined by the open areas of the screen but should follow accurately the image formed on the screen and reproduce it.
These known techniques to produce these coatings are described for example in U.S. Pat. No. 4,292,230 and GE-OS No. 22 36 928.
Screen applied solder masks formed using these known techniques do not hold the exact image during the bake cycle, because a very thin film of the above coating composition exudes from the bulk coating at the baking temperature and spreads or bleeds into window areas. While this exudation is tolerable with conventional printed circuit boards it in unacceptable with high density printed circuit boards because there said exudation can cause poor solderability.
It is therefore the object of the invention to find a way for producing coatings that serve as solder masks on printed circuit boards, where the bleeding in during the baking cycle is inhibited.
It is another object of this invention to produce a coating that has the advantages of the prior art coatings.
It is still another object of the invention to provide a method that is compatible to a large extent with the known methods and affords only minor additional apparative and procedural expenses.